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Sunday, May 15, 2016

#345 Semiconductor material engineering, part. III

Besides affecting width of the energy gap, distribution of energy states within the energy bands of semiconductor affects also the mechanism of electron transition from the higher energy levels in the conduction band to the lower energy levels in the valence band in the process of charge carriers recombination.


In the case of semiconductors featuring direct bandgap the energy released in the process of electron recombination is in the form of light (photon) emitted by the material. In contrast, in the indirect bandgap semiconductors the same energy is released into semiconductor crystal mostly in the form of vibrational energy (phonon).


This inherent characteristic of the energy gap is predetermined for any semiconductor featuring set chemical composition. But when you start altering chemical compositions of some compound semiconductor the change of bandgap’s type from direct to indirect (or vice versa) is entirely possible.


Taking GaAs as an example, but not in combination with Al as in the previous blog, but with phosphorous (P). A peculiar property of the Ga-As-P material system is that at certain composition between GaAs and GaP the energy gap changes form direct to indirect making  material which is initially suitable for  light emitting devices  to the one which is entirely ineffective in this application.


And that’s yet another example of what manipulation of the chemical composition of semiconductor can bring about.


Posted by Jerzy Ruzyllo at 12:33 PM | Semiconductors | Link

Sunday, May 8, 2016

#344 Semiconductor material engineering, part II.

In addition to changing dopant concentration and by doing so changing electrical conductivity of semiconductor (see previous blog), chemical composition of semiconductor can be altered for the purpose of changing its bandgap Eg (width of the energy gap). This practice, known as “bandgap engineering” is commonly employed to form highly complex III-V compound semiconductors material systems used in engineering of high-speed transistors and light emitters.


Take for instance Al-Ga-As material system. By changing Al fraction x in AlxGa1-xAs, material transitions from gallium arsenide GaAs (x = 0) to aluminum arsenide AlAs (x = 1). The bandgap changes in the process from Eg = 1.42 eV to  Eg = 2.16 eV for AlAs.  

Posted by Jerzy Ruzyllo at 07:50 PM | Semiconductors | Link

Sunday, April 24, 2016

#343 Semiconductor material engineering, part. I

As a follow up to the previous post discussing the role of alterations of composition of solids a few remarks regarding specifically semiconductors (see also blog #341).


First and foremost it needs to be emphasized that with an exception of the very specialized p-n junctions which included intrinsic (undoped) “i” component (p-i-n junctions), all semiconductor devices are manufactured used doped material. In fact our ability to control concentration of alien elements in the host material is a foundation of semiconductor device technology. In short, our ability to process diodes and transistors is based on the choice of the dopant type, making semiconductor either p- or n-type, and dopant concentration which allows control over its electrical conductivity.


Importance of dopants in semiconductor device technology is such that unless semiconductor material has elements that can make it either p-type or n-type, its usefulness in the manufacture of diodes and transistor is close to zero. An example of such material is diamond which in theory is an excellent semiconductor, but in practice is hardly used to make commercial devices because no element in the periodic table can effectively make into a device-grade n-type material.


Posted by Jerzy Ruzyllo at 08:54 AM | Semiconductors | Link

Sunday, April 10, 2016

#342 Manipulating chemical composition of solids

Tinkering with chemical compositions of conductors such as metals will do us little good in terms of altering their physical properties to the point where new applications of the material will become possible. As an excellent conductor of electricity, metal can only up to certain point be converted into not-so-good conductor by altering its chemical makeup.


In contrast, fundamental physical properties of some insulators, most notably oxides, as well as semiconductors can be drastically altered by the modification of their chemical composition.


With proper alterations of their chemical composition some oxides can be made conductive, and even superconducting, can be converted from dielectrics to ferroelectrics, can acquire magnetic properties and so on, all under the banner of functional oxides.



Semiconductors story in this regard is equally exciting (see follow up blog).


Posted by Jerzy Ruzyllo at 05:50 PM | Semiconductors | Link

Sunday, April 3, 2016

#341 Different kinds of doping

For years, term "doping" in semiconductor nomenclature meant introduction of alien elements into a given semiconductor material in order to alter its electrical condutivity. For instance, introduction of boron makes silicon assume p-type conductivity while doping with phosphorus makes silicon n-type. And obviously higher the concentration of introduced dopant atoms, higher the electrical condictivity of semiconductor.


These days term "doping" takes on addditional meaning. With a growing interest in an effective control over the electron's spin state (see spintronics) the interest in magnetic semiconductors, i.e. semiconductors displaying  ferromagnetic properties, is also growing. The easiest way to make certain classic semiconductors display some ferromagnetic properties is to dope them with selected alien elements. For instance gallium arsenide, GaAs, will display ferromagnetic properties when manganese (Mn) atoms are introduced into its structure and change it into GaMnAs.


The difference between these two cases of "doping" is that in the former case it takes some 1 atom of dopant per milion atoms of silicon to make a diffference in its electrical conductivity, while  in the latter case manganese doping levels as high as 10% may be needed to see meaningful differences in ferromagnetic properties of GaAs.


Posted by Jerzy Ruzyllo at 12:45 PM | Semiconductors | Link

Sunday, March 20, 2016

#340 Reversing surface "aging"

Good news is that surface aging discussed in the previous blog can be relatively easily reversed by lamp cleaning. However, only light organic contaminants relatively weakly physi-sorbed at the wafer surface can be controlled this way. If allowed to remain on the surface for the long storage times, chemical reactions with moisture from the ambient will convert organic contaminants into heavy organic compound strongly chemi-sorged at the wafer surface. Then, only standard wet clean such as SPM or APM will be able to restore wafer surface to its original condition.

Posted by Jerzy Ruzyllo at 08:01 PM | Semiconductors | Link

Sunday, March 13, 2016

#339 Surface "aging"

Surface contaminants encountered in semiconductor manufacturing can be  added to the surface during wafer processing and  during wafer storage. The latter concerns contaminants from the storage and shipping ambient including clean room air, containers, boxes, and cassettes. Assuming storage is taking place in a particle-free environment the contaminants originating from storage include primarily organic compounds and moisture.


Prolonged exposure to the ambient air during wafer storage results in the surface “aging” process which may interfere with subsequent processes or measurements such as ellipsometric measurements. See Fig. 3 in this paper for the illustration of the dynamics of silicon surface "aging" process as represented by the changes in the value of contact angle as a function of wafers storage time. 


Posted by Jerzy Ruzyllo at 08:13 AM | Semiconductors | Link

Sunday, March 6, 2016

#338 Effect of hydrogen on silicon's electrical conductivity

Questions regarding interactions between alien elements (other than purposly introduced dopants) and electronic properties of semiconductors continue to be relevant. So, it addition to what was already said in this blog two years ago (see post #281), here is a reference that considers a mechanism according to which hydrogen penetrating silicon  may alter its electrical conductivity.

Posted by Jerzy Ruzyllo at 07:29 PM | Semiconductors | Link

Sunday, February 21, 2016

#337 Vertical Slit FET, VeSFET

Among transistor architecures offering possible solutions to the future low-power, high-denisty and highly reconfigurable logic and memory ICs, Vertical Slit FET, or VeSFETcontinues to intrigue me because (i) it offers alternative to transistor scaling solution to the challenges of ultra-low power digital ICs and (ii) it very efficiently integrates transistors in the circuit with multi-level metallization scheme. Not to mention its predicted by several simulations superior performance in terms of, for instance, Ion/Ioff ratio.


It will be interesting to see what will be chip manufacturers reaction to this highly innovative solution.

Posted by Jerzy Ruzyllo at 12:26 PM | Semiconductors | Link

Sunday, February 7, 2016

#336 Thick films

Since in the previous blog I attempted to define a concept of the “thin”-film, a brief comment on what “thick’-films are all about is in order. A film of a solid is referred to as “thick” when its basic physical properties (e.g. resistivity of the conductor) are not significantly different from the physical properties of the same material in the bulk form. Films referred to as thick are in the thickness range typically above 100 µm.


Key difference between thin- and thick-films is in the film deposition technology. While the former use precise methods such as physical vapor deposition (PVD) or chemical vapor deposition (CVD), formation of thick-films involves techniques such as screen printing.


Thick film are not used to make semiconductor devices, and hence, are not a part of broadly understood semiconductor device technology. Thick films are commonly used to make passive elements, mainly resistors, and in general, are an important part of the hybrid IC technology.  


Posted by Jerzy Ruzyllo at 05:40 PM | Semiconductors | Link

‹‹ ›› is the personal blog of Jerzy Ruzyllo. With over 35 years of experience in academic research and teaching in the area of semiconductor engineering (currently holding position of a Distinguished Professor of Electrical Engineering and Professor of Materials Science and Engineering at Penn State University), he has a unique perspective on the developments in this progress driving technical domain and enjoys blogging about it.

With over 2000 terms defined and explained, Semiconductor Glossary is the most complete reference in the field of semiconductors on the market today.

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